Design and implement a TwoSum class. It should support the following operations: add and find.

add - Add the number to an internal data structure.

find - Find if there exists any pair of numbers which sum is equal to the value.

Example 1:

add(1); add(3); add(5);find(4) -> truefind(7) -> false

Example 2:

add(3); add(1); add(2);find(3) -> truefind(6) -> false

 

用hashmap存新加入的元素和频率

add直接把元素加进hashmap。find遍历整个map，分两种情况，如果k和v相同，对应频率要大于1；如果不同，只要map中存在key=v即可。

时间：add - O(1), find - O(N)，空间：O(N)

class TwoSum {    HashMap
     
       map;    /** Initialize your data structure here. */    public TwoSum() {        map = new HashMap<>();    }        /** Add the number to an internal data structure.. */    public void add(int number) {        map.put(number, map.getOrDefault(number, 0) + 1);    }        /** Find if there exists any pair of numbers which sum is equal to the value. */    public boolean find(int value) {        for(Map.Entry
      
        entry : map.entrySet()) {            int k = entry.getKey();            int v = value - k;            if((k != v && map.containsKey(v)) || (k == v && map.get(v) > 1))                return true;        }        return false;    }}/** * Your TwoSum object will be instantiated and called as such: * TwoSum obj = new TwoSum(); * obj.add(number); * boolean param_2 = obj.find(value); */